Laurent Husson

Horizontal subduction zones, convergence velocity and the building of the Andes.

We discuss the relationships between Andean shortening, plate velocities at the trench, and slab geometry beneath South America. Although some correlation exists between the convergence velocity and the westward motion of South America on the one hand, and the shortening of the continental plate on the other hand, plate kinematics neither gives a satisfactory explanation to the Andean segmentation in general, nor explains the development of the Bolivian orocline in Paleogene times. We discuss the Cenozoic history of horizontal slab segments below South America, arguing that they result from the subduction of oceanic plateaus whose effect is to switch the buoyancy of the young subducting plate to positive. We argue that the existence of horizontal slab segments, below the Central Andes during Eocene-Oligocene times, and below Peru and North-Central Chile since Pliocene, resulted (1) in the shortening of the continental plate interiors at a large distance from the trench, (2) in stronger interplate coupling and ultimately, (3) in a decrease of the trenchward velocity of the oceanic plate. Present-day horizontal slab segments may thus explain the diminution of the convergence velocity between the Nazca and South American plates since Late Miocene.

Influence of dynamic topography on sea level and its rate of change

Mantle fl ow likely supports up to 2 km of long-wavelength topographic relief over Earth’s surface. Although the average of this dynamic support must be zero, a net defl ection of the ocean basins can change their volume and induce sea-level change. By calculating dynamic topography using a global mantle fl ow model, we fithat continents preferentially conceal depressed topography associated with mantle downwelling, leading to net seafl oor uplift and ~90 ± 20 m of positive sea-level offset. Upwelling mantle fl ow is currently amplifying positive dynamic topography and causing up to 1.0 m/Ma of sea-level rise, depending on mantle viscosity. Continental motions across dynamic topography gradients also affect sea level, but uncertainty over the plate motion reference frame permits sea-level rise or fall by ±0.3 m/Ma, depending on net lithosphere rotation. During a complete Wilson cycle, sea level should fall during supercontinent stability and rise during periods of dispersal as mantle fl ow pushes continents down dynamic topography gradients toward areas of mantle downwelling. We estimate that a maximum of ~1 m/Ma of sea-level rise may have occurred during the most recent continental dispersal. Because this rate is comparable in magnitude to other primary sea-level change mechanisms, dynamic offset of sea level by mantle fl ow should be considered a potentially signifi cant contributor to long-term sea-level change.

Dynamic topography above retreating subduction zones

Dynamic topography provides a measure of stresses within the Earths interior. Dense slabs induce an upper mantle flow that deflects the surface of the Earth downward above them. By combining a simple theoretical (Stokeslet) model of subduction, gravity modeling, and seismic tomography, I show that a significant fraction (as much as 2000 m) of the topographic variations observed above the Scotia, Mariana, and Hellenic subduction systems appears to be dynamically induced by stresses related to the underlying subduction.

From longitudinal slab curvature to slab rheology

The curvature of a subducting lithosphere is chiefl y controlled by the viscosity ratio between the slab and the surrounding mantle. On the basis of a semi-analytical fl ow model, we explore the rheological dependence of the geometrical response of a viscous slab subjected to toroidal mantle fl ow. Mantle fl ow is excited by slab retreat at a prescribed mean velocity and is iteratively solved for by using a stream function approach, in turn providing the stresses that bend the slab. Comparison between model predictions and geophysical observations of slab curvature gives an average slab-to-mantle viscosity ratio of 45.

Analog models of oblique rifting in a cold lithosphere

New lithospheric analog models of oblique rifting presented here capture the main characteristics of natural oblique rifts and provide insights into the fault evolution, basin segmentation, and mantle exhumation occurring during rift localization. We present two models: one with a preexisting oblique lithospheric weakness (model B) and another with no weakness zone (model A). Both oblique rifts have an obliquity of about 40°. The main results are as follows. (1) The fault populations, especially during the early stages of deformation, are composed of faults that in strike are largely intermediate between rift-parallel and perpendicular to displacement. This fault population is characteristic of oblique rifts observed in previous studies. (2) In later stages, faults parallel to the rift become numerous in both models. Buoyancy forces related to thickness variations in the lithosphere during rift localization play a significant role and control the initiation of rift-parallel faults. (3) During the final stages of extension, in model B the crust is deformed by rift-parallel faults, while in the basins the small-scale deformation pattern is composed of displacement-normal faults. However, in model A, displacement-normal faults tend to accommodate most of the extension, controlling its final stages. They probably also control the formation of the ocean-continent transition, any possible mantle exhumation, as well as the geometry of oceanic accretion centers. These results provide an insight into the possible evolution of the Gulf of Aden conjugate margins, which developed in an oblique context and most probably without any preexisting rift-parallel localizing heterogeneity in the lithosphere.

Subduction with Variations in Slab Buoyancy: Models and Application to the Banda and Apennine Systems

Temporal variations in the buoyancy of subducting lithosphere exert a fi rst-order control on subduction rate, slab dip and the position of the associated volcanic arc. We use a semi-analytic, three-dimensional subduction model to simulate “unforced” subduction, in which trench motion is driven solely by slab buoyancy. Model rates of subduction and model slab dip respond almost immediately to changes in the buoyancy of the subducting lithosphere entering the trench; as more buoyant slab segments correlate with slower subduction rates and steeper slab dip. The results are largely consistent with observations from the Banda and southern Apennine subduction systems, where subduction slowed and ended shortly after the entry of continental lithosphere into the trench. Over a 2 m.y. period, model subduction rates decrease from 70 mm/year to 30 mm/year for the Banda Arc, and from 40 mm/ year to 20 mm/year for the Apennine Arc. Increases in model slab dip and decreases in arc-trench distance are likewise consistent with hypocenter locations and volcanic arc position along the Banda and Sunda arcs. In contrast, a time period of 10 m.y. is needed for model subduction rates to slow to near zero, much longer than the 3 m.y. upper bound on the observed slowing and cessation of trench motion in the Apennine and Banda systems. One possible explanation is that slab break-off, or the formation of large slab windows, occurred during the last stages of subduction, eliminating toroidal fl ow around the slab and allowing the slab to steepen rapidly into its final position

Subducting slabs: Jellyfishes in the Earth's mantle

The constantly improving resolution of geophysical data, seismic tomography and seismicity in particular, shows that the lithosphere does not subduct as a slab of uniform thickness but is rather thinned in the upper mantle and thickened around the transition zone between the upper and lower mantle. This observation has traditionally been interpreted as evidence for the buckling and piling of slabs at the boundary between the upper and lower mantle, where a strong contrast in viscosity may exist and cause resistance to the penetration of slabs into the lower mantle. The distribution and character of seismicity reveal, however, that slabs undergo vertical extension in the upper mantle and compression near the transition zone. In this paper, we demonstrate that during the subduction process, the shape of low viscosity slabs (1 to 100 times more viscous than the surrounding mantle) evolves toward an inverted plume shape that we coin jellyfish. Results of a 3D numerical model show that the leading tip of slabs deform toward a rounded head skirted by lateral tentacles that emerge from the sides of the jellyfish head. The head is linked to the body of the subducting slab by a thin tail. A complete parametric study reveals that subducting slabs may achieve a variety of shapes, in good agreement with the diversity of natural slab shapes evidenced by seismic tomography. Our work also suggests that the slab to mantle viscosity ratio in the Earth is most likely to be lower than 100. However, the sensitivity of slab shapes to upper and lower mantle viscosities and densities, which remain poorly constrained by independent evidence, precludes any systematic deciphering of the observations.

Dynamic ups and downs of the Himalaya

Fast uplift and exhumation of the Himalaya and Tibet and fast subsidence in the foreland basin portray the primary Neogene evolution of the Indian-Eurasian collision zone. We relate these events to the relative northward drift of India over its own slab. Our mantle-flow model derived from seismic tomography shows that dynamic topography over the southward-folded Indian slab explains the modern location of the foreland depocenter. Back in time, our model suggests that the stretched Indian slab detached from the Indian plate during the indentation of the Eurasian plate, and remained stationary underneath the northward-drifting Indian continent. We model the associated southward migration of the dynamic deflection of the topography and show that subsidence has amounted to ∼6000 m in the foreland basin since 15 Ma, while the dynamic surface uplift of the Himalaya amounted to ∼1000 m during the early Miocene. While competing with other processes, transient dynamic topography may thus explain, to a large extent, both the uplift history of the Himalaya and subsidence of its foreland basin, and should not be ignored.

Passive margins getting squeezed in the mantle convection vice

[1] Passive margins often exhibit uplift, exhumation, and tectonic inversion. We speculate that the compression in the lithosphere gradually increased during the Cenozoic, as seen in the number of mountain belts found at active margins during that period. Less clear is how that compression increase affects passive margins. In order to address this issue, we design a 2-D viscous numerical model wherein a lithospheric plate rests above a weaker mantle. It is driven by a mantle conveyor belt, alternatively excited by a lateral downwelling on one side, an upwelling on the other side, or both simultaneously. The lateral edges of the plate are either free or fixed, representing the cases of free convergence, and collision (or slab anchoring), respectively. This distinction changes the upper mechanical boundary condition for mantle circulation and thus, the stress field. Between these two regimes, the flow pattern transiently evolves from a free-slip convection mode toward a no-slip boundary condition above the upper mantle. In the second case, the lithosphere is highly stressed horizontally and deforms. For a constant total driving force, compression increases drastically at passive margins if upwellings are active. Conversely, if downwellings alone are activated, compression occurs at short distances from the trench and extension prevails elsewhere. These results are supported by Earth-like models that reveal the same pattern, where active upwellings are required to excite passive margins compression. Our results substantiate the idea that compression at passive margins is in response to the underlying mantle flow that is increasingly resisted by the Cenozoic collisions.

From the seismic cycle to long-term deformation: linking seismic coupling and Quaternary coastal geomorphology along the Andean Megathrust

Measurement of interseismic strain along subduction zones reveals the location of both locked asperities, which might rupture during Megathrust earthquakes, and creeping zones, which tend to arrest such seismic ruptures. The heterogeneous pattern of interseismic coupling presumably relates to spatial variations of frictional properties along the subduction interface and may also show up in the forearc morphology. To investigate this hypothesis, we compiled information on the extent of earthquake ruptures for the last 500 years and uplift rates derived from dated marine terraces along the South American coastline from central Peru to Southern Chile. We additionally calculated a new interseismic coupling model for that same area based on a compilation of GPS data. We show that the coastline geometry, characterized by the distance between the coast and the trench, the latitudinal variations of long-term uplift rates and the spatial pattern of interseismic coupling are correlated. Zones of faster and long-term permanent coastal uplift, evidenced by uplifted marine terraces, coincide with peninsulas, and also with areas of creep on the megathrust where slip is mostly aseismic and tend to arrest seismic ruptures. We conclude that spatial variations of frictional properties along the megathrust dictate the tectono-geomorphological evolution of the coastal zone and the extent of seismic ruptures along strike.